Our studies in the rabbit have shown that the young appendix is an important site of development of the cells destined to produce protective antibodies. The rabbit appendix has functions similar to those of the avian bursa of Fabricius. In the young rabbit, there is rapid growth and expansion of B lymphocytes in the appendix, and positive and negative selection events occur. We believe that the cells that survive selection in the appendix seed the peripheral lymphoid system and undergo self renewal to maintain the primary repertoire. The data on rabbit have led us to initiate an evaluation of the role of human appendix and GALT in development of the human immune repertoire and mucosal immunity. Hematoxylin and eosin staining of paraffin sections of normal human appendix was used to examine stages of germinal center (GC) development. Observations were also made of rabbit appendix tissues collected at different ages. In both human and rabbit appendix, we found that lymphoid follicles were just beginning to form at birth. Germinal centers became evident at about 1 month in human and at 1 week in rabbit appendix. Most lymphoid development occurred within the first year in the human and within the first 12 weeks in the rabbit appendix. Lymphoid tissue began to atrophy by adulthood in both human and rabbit appendix. The follicles became smaller and fewer in number whereas the connective tissue and smooth muscle increased. The appendix did not entirely involute; germinal centers were still present in advanced age as evident in 5 human samples 75 years or older and a 7 yr-old rabbit specimen. The human appendix shares some characteristics of a ?mammalian bursa equivalent?; it is a B-cell rich organ in the young and the morphology of the follicles and GC changes with age (Dasso et al.,1999 and ms in preparation). A clinical research protocol (#OH99-I-N026) entitled, ?Effect of Appendectomy on Mucosal Immunity In Children? was reviewed and approved by the IRBs of Children?s National Medical Center, and NIAID, NIH. It is under review at Suburban Hospital. The purpose of this pilot study is to determine the effect of appendectomy on total and oral vaccine-specific antibody levels in children. We will measure IgA1, IgA2, IgM, IgG and polio-vaccine- specific Ig levels in fecal and saliva samples from children appendectomized below age 12 and compare these with the levels in children of the same age groups who undergo other major surgery such as for hernias. We are still in the process of accruing subjects. To date we have three experimental and eight control patients enrolled in the initial phase of our study. Two patients have withdrawn citing no reason other than that they have changed their minds. Once enrolled, patients have been very cooperative. We have successfully performed immunologic assays on some of the patient samples. We plan to continue enrolling both study and control patients, and to analyze patient samples for antibody levels as indicated in our protocol. Since the analysis is performed blind, we cannot report preliminary data for treatment and control groups. - Mucosal Immunity, Oral immunization, Gut Associated Lymphoid Tissue, Appendix Development, B-cell Repertoire. - Human Subjects